Many flight vehicles, for example aircraft, obtain forward propulsion from air-breathing engines, i.e. jet engines. In the powerplant system of such an aircraft, air is taken in through a forward-facing air-intake and mixed with fuel, the mixture is ignited, and resultant exhaust gases are expelled, providing forward thrust. Optimisation of the geometric shape of the air intake according to flight condition and engine demand is known to provide performance benefits. Those benefits are particularly significant when flight vehicles are travelling faster than the speed of sound in air; i.e. in the supersonic flight regime.
In order to achieve optimisation of its geometric shape, the entry of a supersonic air intake may be equipped with surfaces angled to the local flow direction, which are used to generate a system of shock waves. The configuration of those features determines the performance efficiency and viability of the intake. For an axi-symmetric intake these “compression” surfaces typically take the form of cone-like centrebodies, whilst for rectangular intakes they generally take the form of a wedge or series of wedges. Actuating the surfaces can provide performance and operating benefits; however, there is significantly increased complexity, mass and cost, especially for axi-symmetric intake configurations.
A flight-vehicle air intake typically comprises a plurality of compression surfaces, typically including a surface having a leading edge, and a cowl having a lip rearward of the body's leading edge.
An oblique shock wave typically forms at the leading edge of an air-intake of a supersonic flight vehicle, and extends towards the intake cowl lip, but, depending on conditions, the shock may extend forwards (upstream) of the cowl lip, or rearwards (downstream) of it. For example, increasing the Mach number of the vehicle will generally cause the shock wave to lean backwards, towards the rear of the vehicle, whilst reducing vehicle Mach number will have the opposite effect. It is desirable to minimise the total pressure loss across the shock system, but if the shock wave extends rearward of the cowl lip, inside the intake, a severe shock loss may occur at the cowl lip. On the other hand, if the shock wave is located ahead of the cowl lip, subsonic air behind the shock wave spills around the outside of the air intake and is accounted as a form of drag. Therefore, it is generally desirable to optimise the position of the shock wave relative to the cowl lip. It may typically be desirable to keep the shock wave on the cowl lip.
Where multiple shocks are present and intersect, an interface vortex sheet can be formed at their intersection, which can cause an instability known as “buzz”. Buzz is a cyclic interaction of the shock system and the flow within the intake, causing instabilities in the engine operation.
In a known supersonic flight vehicle, parts of an air intake are moved mechanically to vary the geometric shape of the intake. The parts moved include in particular those parts that form the internal or external flow compression surfaces and throat profiles.
In an example of a prior-art system (FIG. 1), a powerplant system 5 including an air-breathing engine 7 has an air intake 10, defining an air intake aperture 20, and having a lower leading edge 30. At supersonic speeds, leading edge 30 slows approaching air, causing a leading-edge shock-wave 40.
To reduce the effects of the leading-edge shock wave, the air intake aperture 20 has a lower surface comprising, immediately rearward of its leading edge, a fixed compression surface 15, and, rearward of that, a flap 50. Flap 50 is pivotable and attached adjacent to the fixed compression surface 15, and is also attached to the surface via piston cylinder 60. Flap 50 provides a moveable compression surface 55, which is moved by the action of piston 60 on flap 50.
Piston 60, and hence flap 50, moves from a retracted position, in which flap 50 lies within a recess 65 in fixed compression surface 15, to an extended position, in which flap 50 extends a substantial way across air-intake aperture 20. When flap 50 is in the extended position, a second, flap-induced shock-wave 70 forms from near where flap 50 is pivoted. The presence of the second shock-wave can produce powerplant performance benefits.
When the flight vehicle speeds up, the angle of flap 50 is increased, to counter the movement of the shock wave, and to bring it back to cowl lip 130. Similarly, when the engine mass flow is throttled, the angle of flap 50 is reduced accordingly. Greater efficiency is achieved with a variable geometry.
That approach to providing a variable-geometry air intake adds to the structural weight and complexity of the flight vehicle, which for many applications can severely reduce the net benefit from the variable geometry. Another example of a complex prior-art design is a translating or expanding conical centrebody (rather than flap 50) provided to make variable the geometry of an axi-symmetric intake.
As is well known, a supersonic flow can only be turned at a shock-wave. A shock wave intersecting with a surface can produce a very large pressure change in the boundary layer, causing it to separate from the rest of the flow. Various arrangements are used to avoid this; for example a diverter, in the form of a leading edge lip, may be used to separate it from the rest of the flow, and divert it away from the engine intake. A bleed (in the form of an aperture through which air is blown tangentially along the surface of the intake) may be used to re-invigorate the boundary layer.
In known prior art arrangements, particularly used in Ramjet engines suitable for supersonic propulsion, fuel is injected into the engine to optimise combustion. The angle of fuel injection can be controlled to optimise boundary layer distribution within the engine which in turn improves performance.
It would be advantageous to provide a variable geometry air intake for an air-breathing flight vehicle in which one or more of the aforementioned disadvantages is eliminated or at least reduced.